WO2012032354A1

WO2012032354A1 - Separation system

Info

Publication number: WO2012032354A1
Application number: PCT/GB2011/051687
Authority: WO
Inventors: Stephen Goodwin; Terence Mccarthy; Geraint Catley
Original assignee: Aquabio Limited
Priority date: 2010-09-09
Filing date: 2011-09-09
Publication date: 2012-03-15
Also published as: GB201014993D0

Abstract

A separation system comprising a separation device having an inlet arranged to allow a flow of material into the separation device, a first outlet arranged to outlet a flow of retentate and a second outlet arranged to outlet a flow of permeate, the system further comprising a recirculation pump arranged on the inlet of the separation device and flow detection means operable to detect a flow of permeate in the second outlet, characterised in that the flow detection means is adapted to provide flow information to control means, which control means are arranged and operable to control the speed of the recirculation pump in response to the said flow information.

Description

Separation System

The present invention relates to a separation system, in particular to a system for separating water from activated sludge, being either aerobic or anaerobic, in a process for treating polluted water. The invention also extends to a method of separating water from activated sludge and to a backflushing system and method for such a separation system.

The separation of liquids from liquid/solid slurries may be undertaken by many different methods known in the art. One method of separation, of particular use in separating water from a contaminated source, is cross-flow filtration (also known as tangential flow filtration) in which a slurry feed flows tangentially across the surface of a filter, allowing permeate to flow out of the filter, while the bulk of the retained material, the retentate, continues past the filter. In such systems, it is also known to have a reactor tank in which the feed water to be treated is mixed and reacted with activated sludge to form a slurry feed prior to separation.

A common arrangement for cross-flow filtration involves the use of a tubular membrane. In use, in one scenario, a feed liquor (slurry) passes through the tubular membrane at pressure and permeate is leached across the membrane filter to an outer side of the tubular membrane and collected separately. Alternatively, the liquor may be passed tangentially over the tubular membrane at pressure, with permeate being sucked into the lower pressure interior of the tubular membrane. Both systems comprise an inlet for feed liquor, an outlet for retentate and an outlet for the permeate. In large scale separation systems, it is known to house a number of tubular membranes clustered together within a large outer tube module.

The permeate is usually discharged from the membrane system to a collection tank or direct to a consented discharge point or sometimes to different locations throughout the operating time of the process depending on the further treatment or use of the treated water. The driving force for the flow of permeate across the membrane is the differential pressure from the inside to the outside of the membrane, known as the trans-membrane pressure or TMP. An increase in the permeate side pressure causes a reduction in TMP and hence reduces the flux across the membrane where flux is the flow of permeate relative to the membrane surface area. Two main factors tend to affect the pressure on the permeate side and therefore affect the flux at the membrane; firstly the static pressure of the discharge point or permeate collection tank; secondly the frictional pressure losses in the permeate discharge pipe. For a membrane separation process comprising more than one membrane system the frictional pressures losses in the permeate discharge pipe are variable depending on the number of membrane systems discharging into the same pipe; the greater the number of membrane systems operating, the greater the pressure on the permeate side. Therefore a drawback of the described system is that the permeate flow per system is reduced as more systems are operated and also if the discharge destination is modified to one with a higher static pressure. A schematic view of such a prior art separation system is shown in Figure 1 and described in more detail hereunder.

One solution to the above described problem is to include a pump to draw permeate out of the membrane. The speed of the pump is adjusted to control the permeate flow from the membrane. However, it is important to note that the relationship between flux and TMP is not always constant since the filterability is also dependant on the rheological properties of the activated sludge in the reactor tank. These properties can change over time, dependant on the health of the activated sludge i.e. if the correct sustenance and environmental conditions are not present the biological culture tends to exhibit different rheological properties which are detrimental to filterability. Therefore the drawback of using a pump to control permeate flow is that the selected flow setpoint may not be sustainable at that time depending on the filterability of the activated sludge and may lead to increased fouling of the pores of the membrane.

It is an object of aspects of the present invention to provide a solution to the above mentioned or other problems.

According to a first aspect of the present invention there is provided a separation system comprising a separation device having an inlet arranged to allow a flow of material into the separation device, a first outlet arranged to outlet a flow of retentate and a second outlet arranged to outlet a flow of permeate, the system further comprising a recirculation pump arranged on the inlet of the separation device and flow detection means operable to detect a flow of permeate in the second outlet, characterised in that the flow detection means is adapted to provide flow information to control means, which control means are arranged and operable to control the speed of the recirculation pump in response to the said flow information. The present invention avoids the drawbacks described above by controlling the permeate flow at the downstream side of the or each separation device by adjusting the speed of the recirculation pump upstream of the or each separation device. The separation device may comprise one or more separation units. The or each separation unit may comprise a cross flow filtration device. The or each separation unit may comprise a membrane filter, preferably a tubular membrane filter.

The sum of the surface areas of the or each membrane filter may be referred to herein as the "membrane surface area".

Preferably, the separation system comprises a feed tank, which may be a reactor tank. The feed tank may comprise means to inject one or more gases thereinto. In an alternative embodiment it may not be necessary to add gases, such as with anaerobic sludge.

An inverter may be used to control the speed and hence the flow output of the recirculation pump. Controlling the speed of the recirculation pump affects the velocity of the activated sludge passing along the or each separation device membrane tubes, known as the cross-flow velocity. With increasing cross-flow velocity comes increasing pressure on the inside of the membrane tubes and hence increasing TMP and flux. In this way the adjustment of the speed of the recirculation pump can be used to control the permeate flow. If the permeate flow is increased by a pump on the permeate side drawing from the membrane, while the cross-flow inside the membrane remains the same, the concentration of activated sludge at the membrane surface is increased leading to increased filter cake build-up and hence increased blocking of the membrane pores. The advantage of controlling the permeate flow using the speed of the recirculation pump is that cross-flow is increased at the same time as the permeate flow is increased and therefore the concentration at the membrane surface remains the same or in some cases reduces which helps to reduce fouling. With increasing cross-flow also comes increasing scouring of the membrane surfaces which also helps to reduce fouling. A further benefit of controlling permeate flow using the speed of the recirculation pump is exhibited in reduced energy usage and in optimum sizing of the membrane surface area. The recirculation pump speed is preferably operated, in use, at sufficient speed to match the required permeate flow. Accordingly, in such a scenario, the energy use is minimised in relation to the throughput of the plant. This is particularly beneficial where varying flow throughput is required because membrane surface area can be selected to match the average flow throughput at low energy usage and peak flows can be accommodated by an increase in recirculation speed without the need for additional membrane surface area. In addition, the relationship between permeate flux and TMP also varies with temperature; increasing temperature leads to increasing permeate flux for the same TMP. Hence at higher operating temperatures, the same permeate flux can be achieved at lower TMP. Therefore the present invention can also take advantage of higher operating temperatures by operating at lower recirculation pump speed and hence lower energy usage.

Preferably, the system further comprises a permeate pump arranged on the second outlet. Preferably, the permeate pump is arranged to control pressure at the permeate side of the membrane. Preferably, a flow detection means is arranged on the second outlet upstream or downstream of the permeate pump.

Additionally, pressure detection means may be arranged upstream of the permeate pump. Preferably, an output signal of the pressure detection means may be provided to control means, which may control the speed of the permeate pump in response to the output signal of the pressure detection means. In this manner, a predetermined pressure at the permeate side of the separation device may be generally maintained.

Advantageously, this means that regardless of the static pressure at the permeate discharge destination or the number of separation units in the separation device discharging permeate into the same pipework, the pressure experienced on the permeate side of the separation device remains generally constant and hence permeate flow remains generally constant given the same internal pressure conditions and the same filterability of the activated sludge. The additional energy required to achieve this permeate pressure control is minimal since the additional suction pressure required is normally as low as 0.1 bar. The benefit of permeate pressure control can be greater than 10% of the flux performance. Since this control is used in conjunction with the permeate flow being controlled by the speed of the recirculation pump, this benefit in flux performance is manifested in reduced recirculation speed to achieve the same permeate flow and hence reduced energy.

In addition to the above described drawbacks and solutions, an additional problem is introduced by the varying speed of the recirculation flow, especially in tubular membranes. A possible problem with tubular type membranes is that debris in the feed media or over-concentration of the activated sludge through the separation process can lead to blockages of the membrane tubes. Debris is usually removed from the polluted water supply prior to entering the reactor vessel and in addition further debris filtration is provided at the feed of each membrane system in the form of a basket type strainer or similar filtration device. Notwithstanding these preventative measures, debris can still cause blockage problems with tubular membranes. Over- concentration of the activated sludge can occur if the cross-flow of activated sludge is relatively low and the quantity of permeate extracted high. If a significant number of membrane tubes are blocked by these mechanisms the differential pressure between the inlet and outlet of the membranes will increase. However since the recirculation pump is adapted to operate at a varying speed and hence produce varying pressure conditions, there is a difficulty in determining a fixed high inlet pressure or a high differential pressure which would indicate that tube blockage is occurring.

Preferably, the separation system further comprises flow detection means operable to provide flow information in the separation device. Measurement of the flow in the separation device and the flow of permeate may be used in combination to calculate a degree of concentration of solids, preferably activated sludge, as it passes through the separation system. This may be referred to as Biomass Concentration Factor or BCF.

This calculated degree of concentration of solids may be used to initiate a warning alarm, for example when a high BCF value is reached, and alternatively or additionally alarm and shutdown of the process when a still higher BCF value is reached.

Preferably, a hydraulic calculation may also be used to determine the expected pressure loss between the inlet and outlet of the separation device for any given cross-flow velocity. The relationship of velocity and pressure derived from these calculations may be expressed as a formula such that the expected pressure loss may be calculated by the control system in real time from the cross-flow velocity derived by the flow detection means of the system. The coefficients within the formula are made available at the control means to allow for tuning of the theoretical formula from empirical data. The actual pressure at the inlet and outlets of the separation devices may be measured by pressure measuring devices and the actual pressure loss may be calculated by control means. The difference between the actual pressure loss and the expected pressure loss may be calculated as a pressure loss discrepancy expressed as a percentage error or as a pressure difference value above or below the expected pressure loss. The calculated value of the pressure loss discrepancy may be used to initiate a warning alarm, for example when a high pressure discrepancy value is reached and alternatively or additionally alarm and shutdown of the process when a still higher pressure discrepancy value is reached.

According to a further aspect of the present invention there is provided a method of separating a liquid from a liquid/solid slurry, the method comprising inputting a flow of material into a separation device via an inlet, the separation device also having a first outlet and a second outlet, outputting a flow of permeate via the second outlet arranging flow detection means on the second outlet, the flow detection means being operable to detect a flow of permeate in the second outlet, providing flow information detected by the flow detection means to control means, controlling the speed of a recirculation pump arranged on the inlet of the separation device in response to the flow information.

In addition, the above described inventions are particularly beneficial when applied in conjunction with intermittent backflushing of the separation device.

Therefore, according to a further aspect of the present invention there is provided a backflushing method, comprising the steps of

i) backflushing a fluid through a separation device of a separation system by causing the said fluid to flow through a second outlet of the separation device, the second outlet being arranged in normal operation to outlet a flow of permeate from the separation device and, the fluid being backflushed out of the separation device through a first outlet or an inlet of the separation device; the first outlet being arranged in normal operation of the separation device to outlet a flow of retentate, and the inlet being arranged in normal operation of the separation device to allow a flow of material into the separation device; ii) optionally introducing a cleaning agent into the separation device and leaving the cleaning agent in the separation device for a residence period, R, by temporarily ceasing the backflushing;

iii) and, if a cleaning agent has been introduced, rinsing the cleaning agent out of the separation device.

The fluid may be water or may be permeate already separated from a liquid/solid slurry by the separation device.

In one embodiment, the backflushing method further includes a preliminary step of pre-rinsing the separation device, preferably by introduction of a rinsing fluid, such as water or permeate, for example, preferably via the recirculation pump, to displace activated sludge from the separation device.

Preferably, while introducing the cleaning agent into the separation device and during the residence time R, the recirculation pump is substantially inactive. Preferably, the fluid is backflushed out of the separation device through the first outlet.

Preferably, the cleaning agent is introduced into the backflush fluid and hence carried into the separation device. Where the method of the present aspect is used on separation systems comprising multiple separation devices, the method of backflushing may be undertaken on each separation device individually, or on several or all of the separation devices of the separation system simultaneously. Preferably, the residence period, R, is greater than 30 seconds, more preferably greater than 1 minute and most preferably greater than 5 minutes. The residence period, R, may be less than 90 minutes, preferably less than 60 minutes and most preferably less than 30 minutes. In one embodiment, it is envisaged that step (ii) may be repeated with different cleaning agents. The cleaning agent may comprise any cleaning agent suitable for such a purpose, for example, sodium hypochlorite solutions of suitable concentrations as are known in the art.

By the term normal operation when referring to the separation system or separation device, it is meant the use of the system or device to separate a liquid from a liquid/solid slurry, such as described herein with regard to the above aspects.

The separation system of the first aspect may be combined with the backflushing method of the present aspect.

In a conventional cross-flow type membrane system, high cross-flow velocities are used to provide fouling control by the scouring of the membrane surfaces; thus reducing the build-up of a cake layer. The separation system of the present invention may be arranged and operable to be backflushed preferably by pumping liquid in a reverse direction through the second outlet of the separation device and out of the first outlet or the inlet of the separation device. This reverses the normal flow of liquid through the separation device and hence removes blockages from the separation device, which may be in pores of a membrane for example. The backflushing described above provides fouling control for the membranes and thus replaces the requirement for high cross-flow velocities. The backflushing is normally carried out on an intermittent basis using a separate backflush pump drawing from a permeate collection tank or a separate backflush water tank. The backflushing may occur on one module in the series at a time and may occur at the same time as normal operation for the remaining modules. Hence the backflush water passes through the separation device and joins the inlet/first outlet flow which may be of activated sludge, which is discharged back to a reactor vessel.

In addition, chemical dosing can be applied to the backflush flow to deliver cleaning fluid to the separation device on an intermittent basis. The addition of chemicals can be applied as part of normal backflushing or most beneficially as part of an automated cleaning procedure. The latter involves shutting down of the separating device, flushing out of biomass using the permeate or other clean water source and then introduction of the cleaning fluid in the backflush system with no recirculation through the separation device modules. The cleaning chemical may be allowed to soak for a time period and then may be automatically flushed away using the backflush system. If more than one membrane system is arranged, such that one system must be cleaned using the above described backflush cleaning arrangement whilst other systems are still in use for the separation process, it may not be advantageous for the backflush water to exit the membrane system via the inlet or first outlet of the membranes since this route may join other pipe arrangements which may contain the slurry feed or retentate stream and hence possible contamination of the membrane system to be cleaned may occur. In such a scenario, it may be advantageous to arrange for the backflush water to exit the system via the permeate outlet of other membrane filters within the system and hence exit via the permeate discharge arrangement.

Following these operations the separation device is started up automatically in normal operation mode.

As used herein, the term "speed of the recirculation pump" refers to the operating speed of the recirculation pump, that is, the rotating speed of the pump.

An additional aspect of the present invention is the possible use of the permeate pump to provide both the permeate pressure control function and the backflushing function. This involves the addition of a divert line from the permeate pump discharge to the backflushing header incorporating a flowmeter and two additional control valves; one controlling a bypass around the pump and one controlling the normal discharge route from the pump. When backflushing is initiated the control valve in the pump bypass line will open to allow flow around the pump to the normal permeate discharge route. The control valve at the discharge of the pump will close to prevent flow from the pump going to the normal discharge. Instead the flow will be diverted to the backflush header where the sequenced operation of the backflush control valves will allow the backflush water into each module in turn. During this sequence, the permeate pump will come under the control of the backflush flowmeter to achieve the required backflush flow setpoint. The backflush flow requirement for one module is always less than the total permeate flow derived from the other modules and hence the excess flow will bypass the permeate pump and pass to the normal discharge route. This means that there is always the sufficient flow from the other modules to satisfy the backflushing requirements of each module. Hence the invention achieves the following benefits; eliminates the requirement for a separate backflush pump and associated motor control, eliminates any vessels for the storage of backflush water and associated level controls and improves the modular design of each membrane system i.e. each membrane system can be provided with its own in-built backflushing system.

Therefore, according to a further aspect of the present invention there is provided a separation and backflushing system, the system comprising a separation system, the separation system comprising at least one separation device having an inlet arranged to allow a flow of material into the separation device, a first outlet arranged to outlet a flow of retentate and a second outlet arranged to outlet a flow of permeate, the system further comprising a recirculation pump arranged on the inlet of the separation device, and a permeate pump arranged on the second outlet, wherein the permeate pump is operable in a first mode of operation to pump permeate from the second outlet and away from the separation device, and wherein the permeate pump is operable, in a second mode of operation to pump fluid back into the separation device to perform a backflushing routine.

Preferably, in the second mode of operation the permeate pump is operable to pump fluid back into the second outlet of the separation device.

Advantageously, in this manner, the permeate pump can also be utilised as the backflush pump and the requirement for a separate storage vessel for backflush feed water is eliminated.

All of the features contained herein may be combined with any of the above aspects and in any combination.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which: Figure 1 shows a schematic view of a prior art separation system;

Figure 2 shows a schematic view of a separation system according to the invention; and Figure 3 shows a schematic view of a separation system including a backflushing system for a separation system. Referring firstly to figure 1 there is shown a schematic view of a prior art separation system 102 comprising a reactor tank 104 containing the slurry 106 to be separated. The slurry 106 contains particulates such as biomass. Also the tank 104 is fitted with a system 108 that is operable to add other gases such as air, carbon dioxide, oxygen, methane or a substantially inert gas, such as nitrogen, for example, to the slurry 106. It will be appreciated by a person skilled in the art that if the slurry is an anaerobic slurry, then the input of gases may not be required. The separation system 102 also comprises a pump 1 10 situated between the tank 104 and a series of two membrane filter units 1 12, 1 14. The membrane filter units 1 12, 1 14 are arranged in series and comprise an array of tubular membranes situated within an outer tubular housing. In use, the separation system 102 operates as follows. Slurry 104 is pumped via the pump 1 10 through the membrane filter units 1 12, 1 14. The first membrane filter unit 1 12 separates an amount of permeate (removed water) from the liquid/solid feed slurry 104, and the remaining retained slurry (the retentate) is passed into the second membrane filter unit 1 14. The second membrane filter unit 1 14 operates in the same manner as the first membrane filter unit 1 12 and separates further permeate from the retentate stream.

The permeate separated from the two membrane filter units 1 12, 1 14 is then discharged for further processing 1 16, while the retentate is passed back into the tank 104 as shown by arrow 1 18.

The system is controlled by the operation of pump 1 10 at a fixed speed to provide a fixed recirculation flow through the membrane filter units. However, the system 102 suffers from several disadvantages as discussed above. For example, the pressure on the permeate is vulnerable to change, which affects the TMP of the filter units 1 12, 1 14, thereby causing less efficient separation.

Referring now to figure 2 there is shown a separation system 202 according to the invention. The separation system 202 has many features in common with the system 102 and comprises a reactor tank 204 containing slurry 206 to be treated. The slurry 206 contains particulates such as biomass. Also in the tank 204 is fitted with a system 208 that is operable to add other gases such as air, carbon dioxide, oxygen, methane or a substantially inert gas, such as nitrogen, for example to the slurry 206. It will be appreciated by a person skilled in the art that other gases may be added, as required by the particular slurry.

The separation system 202 also comprises a recirculation pump 210 situated between the tank 204 and a series of two membrane filter units 212, 214. It will be appreciated by one skilled in the art that the system 202 may comprise one or several membrane filter units. The membrane filter units 212, 214 are arranged in series and comprise an array of tubular membranes situated within an outer tubular housing.

However, in contrast to the system 102, the system 202 also comprises a second pump 220 connected to the permeate flow 216 and a flow meter 222, also connected to the permeate flow 216, downstream of the pump 220 and operable to measure the flow of the permeate. The flow meter 222 could, in an alternative embodiment, be added upstream of the pump 220. The permeate pump 220 receives signals from a pressure monitor 221 upstream of the pump 220, which are used to regulate the speed of the permeate pump.

In use, the separation system 202 generally operates in a similar manner to the system 102. However, the flow meter 222 is arranged and operable to detect flow information of the permeate and to provide this information to control means, which control means is adapted to control the speed of the recirculation pump 210 in response to the detected flow of the permeate.

The benefit of the control of permeate flow by varying the speed of the recirculation pump 210 is exhibited in reduced energy usage and in optimum sizing of membrane surface area. The recirculation pump 210 is operated at sufficient speed to match the required permeate flow, hence the energy use is minimised in relation to the current throughput of the system 202. This is particularly beneficial where varying flow throughput is required; membrane surface area can be selected to provide the optimal flow throughput at low energy usage and peak flows can be accommodated by an increase in recirculation speed without the need for additional membrane surface area. In addition, the relationship between permeate flux and TMP also varies with temperature; increasing temperature leads to increasing permeate flux for the same TMP. Hence at higher operating temperatures, the same permeate flux can be achieved at lower TMP. Therefore the present invention will also take advantage of higher operating temperatures by operating at lower recirculation pump speed and hence lower energy usage. By proceeding in this manner and controlling the recirculation pump rather than the permeate pump, for example, the problems associated with using the permeate pump, as discussed in the preamble above, are thereby alleviated.

Referring now to figure 3 there is shown a schematic view of a separation system 302 including a backflushing system 303

The backflushing system comprises input lines 304 which deliver permeate from membrane filters. In the example shown in figure 3, the system is used in conjunction with a separation system comprising two membrane filter units, and hence comprises two input lines 304.

Each input line 304 enters a permeate header line 306, via a valve 308, 309.

The system 302 further comprises a pump 316, which can be used to pump the permeate through to the permeate discharge 310 via a pump discharge valve 318. When pump 316 is in normal permeate pumping duty, control valve 312 remains closed. This valve is opened if pump 316 is not in use or during a backflush sequence as described below. The system 302 also comprises a permeate pressure measuring device 322 upstream of the pump 316 which is used to control the speed of the pump 316 as described above.

The pump 316 is also connected via a backflush header 324 and valves 320, 321 back to the input lines 304. The backflush header 324 also includes a flow measuring device 326.

In use, the system 303 is operable to backflush a separation system, such as the system 302, by pumping liquid in a reverse direction across the membrane surface. This is achieved by opening one valve 320 (while other valve 321 remains closed) and closing the corresponding valve 308 (while other valve 309 remains open). Valves 308, 309 (and similarly 320, 321 ) can also be replaced by a single 3-way valve to achieve the same duty. This allows for the transfer of backflush water into one membrane filter while allowing the flow of permeate out of the other membrane filter(s). Where the flow of permeate from the other membrane filter(s) is higher than the required backflush flow, this is accommodated by the opening of valve 312, hence allowing excess permeate flow to continue to the permeate discharge 310. In addition control valve 318 is closed such that the flow from pump 316 is directed to the backflush header 324. During the backflush the pump 316 comes under the control of the backflush flow measuring device 326 to achieve the required backflush flow and is not controlled by the pressure measuring device 322. A separation system and method according to the present invention alleviates the problems of the prior art and provides a more reliably controllable system, thereby providing more controllable and efficient separation. In addition, the system and method allows for the operation of the separation device at substantially lower energy usage than the prior art.

A backflushing method according to the present invention includes a static cleaning period to allow a cleaning agent to "soak" prior to removal. In this manner a more efficient cleaning process is achieved. Furthermore, the backflushing system of the present invention has the advantage that it does not require a separate backflush pump nor a separate backflush water storage vessel, thereby further increasing the efficiency of the system.

In one embodiment, the separation method may be used in conjunction with a feed or reactor tank that has an elevated fluid level with regard to the separation device. The additional height of liquid/solid slurry above the separation device is operable to provide additional static pressure which thereby provides additional TMP and hence increases the rate of permeate flow. In this manner, increased permeate flow may be achieved without additional recirculation flow from the recirculation pump and therefore provides higher permeate flow without additional energy use.

In one embodiment, there is provided a method for controlling the permeate pressure via the use of a control valve or other similar pressure control device. The control valve or pressure control device may be used in replacement of or in addition to the above described permeate pump to control the permeate pressure. The control valve or pressure control device may be controlled by the same pressure detection means as the permeate pump or by a separate pressure detection means. The pressure detection means may be preferably arranged upstream of the control valve or pressure control device. Preferably, an output signal of the pressure detection means may be provided to control means, which may control the flow through the control valve or other aspect of the pressure control device in response to the to the output signal of the pressure detection means. In this manner, a predetermined pressure at the permeate side of the separation device may be generally maintained.

Advantageously this means that the TMP of the separation device may be controlled to control the rate of permeate flow across the membrane surface and hence control the rate of fouling. In addition, this control may be used to limit the rate of permeate flow and hence prevent over-concentration of the retentate stream. Such a permeate pressure throttling control is advantageous when the pressure on the inlet and first outlet of the separation device is high which may lead to the TMP being too high and thus permeate flow being too high. This case may occur if a feed or reactor tank with an elevated fluid level is used.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1 . A separation system comprising a separation device having an inlet arranged to allow a flow of material into the separation device, a first outlet arranged to outlet a flow of retentate and a second outlet arranged to outlet a flow of permeate, the system further comprising a recirculation pump arranged on the inlet of the separation device and flow detection means operable to detect a flow of permeate in the second outlet, characterised in that the flow detection means is adapted to provide flow information to control means, which control means are arranged and operable to control the speed of the recirculation pump in response to the said flow information.

2. A separation system according to claim 1 , wherein the separation device comprises one or more separation units.

3. A separation system according to claim 2, wherein the or each separation unit comprises a cross flow filtration device.

4. A separation system according to either of claim 2 or claim 3, wherein the or each separation unit comprises a membrane filter, preferably a tubular membrane filter.

5. A separation system according to any preceding claim which further comprises a feed tank, which may be a reactor tank.

6. A separation system according to any preceding claim, wherein inverter is used to control the speed and hence the flow output of the recirculation pump.

7. A separation system according to any preceding claim which further comprises a permeate pump arranged on the second outlet.

8. A separation system according to claim 7, wherein the permeate pump is arranged to control pressure at the permeate side of the membrane.

9. A separation system according to claim either of claim 7 or claim 8, wherein flow detection means is arranged on the second outlet upstream or downstream of the permeate pump.

10. A separation system according to any of claims 7 to 9, wherein pressure detection means are be arranged upstream of the permeate pump.

1 1 . A separation system according to claim 10, wherein an output signal of the pressure detection means is be provided to control means, which may control the speed of the permeate pump in response to the output signal of the pressure detection means.

12. A separation system according to any preceding claim, which further comprises flow detection means operable to provide flow information in the separation device.

13. A separation system according to claim 12, wherein measurement of the flow in the separation device and the flow of permeate may be used in combination to calculate a degree of concentration of solids, preferably activated sludge, as it passes through the separation system.

14. A separation system according to claim 13, wherein the calculated degree of concentration of solids is used to initiate a warning alarm.

15. A method of separating a liquid from a liquid/solid slurry, the method comprising inputting a flow of material into a separation device via an inlet, the separation device also having a first outlet and a second outlet, outputting a flow of permeate via the second outlet arranging flow detection means on the second outlet, the flow detection means being operable to detect a flow of permeate in the second outlet, providing flow information detected by the flow detection means to control means, controlling the speed of a recirculation pump arranged on the inlet of the separation device in response to the flow information.

16. A backflushing method, comprising the steps of

i) backflushing a fluid through a separation device of a separation system by causing the said fluid to flow through a second outlet of the separation device, the second outlet being arranged in normal operation to outlet a flow of permeate from the separation device and, the fluid being backflushed out of the separation device through a first outlet or an inlet of the separation device; the first outlet being arranged in normal operation of the separation device to outlet a flow of retentate, and the inlet being arranged in normal operation of the separation device to allow a flow of material into the separation device;

ii) optionally introducing a cleaning agent into the separation device and leaving the cleaning agent in the separation device for a residence period, R, by temporarily ceasing the backflushing;

17. A backflushing method according to claim 16, wherein the fluid is water or permeate already separated from a liquid/solid slurry by the separation device.

18. A backflushing method according to claim 16 or claim 17, which further includes a preliminary step of pre-rinsing the separation device, preferably by introduction of a rinsing fluid, such as water or permeate, for example, preferably via the recirculation pump, to displace activated sludge from the separation device.

19. A backflushing method according to any of claims 16 to 18, wherein the fluid is backflushed out of the separation device through the first outlet.

20. A backflushing method according to any of claims 16 to 19, wherein while introducing the cleaning agent into the separation device and during the residence time R, the recirculation pump is substantially inactive.

21 . A backflushing method according to any of claims 16 to 20, wherein the cleaning agent is introduced into the backflush fluid and hence carried into the separation device.

22. A backflushing method according to any of claims 16 to 21 , wherein where the method is used on separation systems comprising multiple separation devices, the method of backflushing may be undertaken on each separation device individually, or on several or all of the separation devices of the separation system simultaneously.

23. A backflushing method according to any of claims 16 to 22, wherein the residence period, R, is greater than 30 seconds.

24. A separation and backflushing system, the system comprising a separation system, the separation system comprising at least one separation device having an inlet arranged to allow a flow of material into the separation device, a first outlet arranged to outlet a flow of retentate and a second outlet arranged to outlet a flow of permeate, the system further comprising a recirculation pump arranged on the inlet of the separation device, and a permeate pump arranged on the second outlet, wherein the permeate pump is operable in a first mode of operation to pump permeate from the second outlet and away from the separation device, and wherein the permeate pump is operable, in a second mode of operation to pump fluid back into the separation device to perform a backflushing routine.

25. A separation and backflushing system according to claim 24, wherein, in the second mode of operation, the permeate pump is operable to pump fluid back into the second outlet of the separation device.